Generic metal carriers for monolithic catalyst are described, e.g., in DE-OS 40 25 434 or DE-PS 40 16 276 as well as in EP 0 245 736 and EP 0 245 737. The metal-carrier matrix consists, usually of a ferritic Cr--, Al-- containing iron alloy which is resistant to high temperature with further important constituents, such as traces of Ce, Zr, Y, rare-earth, alkaline earth and/or alkaline metals, the purpose of which is to increase the temperature resistance and to prevent flaking of the protective layer of oxide on the steel.
According to the aforementioned patent applications and laid-open specifications the metal-carrier matrix is produced by intertwining one or several sheet-metal stacks consisting of corrugated or alternately corrugated and flat metal strips and the matrix is subsequently inserted into a jacket tube. This is followed by joining the metal strips to the jacket tube at the points where they contact the inner surfaces of the jacket tube in order to secure the metal matrix in the jacket tube by means of an appropriate joining process.
A joining process which is relied upon uses the costly method of high-temperature soldering in a high vacuum at temperatures above 1100.degree. C. Different welding processes such as electron-beam welding (DE-OS 27 20 322) and laser-beam welding (DE-OS 27 27 967) are described with respect to metal carriers having a spirally coiled matrix.
The metal strips may be obtained from long metal ribbons by cutting these ribbons to the desired lengths.
The metal carriers thus produced are then coated with an activity-enhancing intermediate layer of finely dispersed metal oxides having a large surface area and promoters. This coating layer is generally referred to as the washcoat, and it supports catalytically active precious metals. The metal carriers are then subsequently welded (e.g., at the ends of the carrier's jacket tube) into the exhaust system of an internal combustion engine.
An advantageous feature of metal carriers or monolithic catalysts with a metal carrier produced by high-temperature soldering is the high mechanical stability of the metal carriers. Outward displacement or telescoping of the metal-carrier matrix from the jacket tube due to the exhaust gas pressure and stresses arising from changes of temperature is typically not observed with monolithic catalysts such as these. This stability is due to the fact that the free ends of each individual strip of metal are firmly connected with the jacket tube.
However, the coating of the finished metal carrier produced in accordance with the above-described method results in highly non-uniform coating thicknesses. The non-uniform coating thickness is due to the fact that in the corners along the contact line between two neighboring metal strips, which usually form an acute angle but may also be right-angled, the capillary forces cause more coating material to accumulate in these areas than on the free surfaces. This non-uniform coating of the matrix results in increased consumption of coating material and precious metals. Coating after the metal carriers have been finished thus constitutes an additional, costly production step.
Another process for manufacturing monolithic catalysts with a metal carrier, which is in competition with the production process described above, commences with the application of the activity-enhancing intermediate layer and the precious metals on flat and/or corrugated metal strips. Depending on the way the strips are embossed, the application step is followed by spiral coiling of corrugated strips only or both a flat and a corrugated strip. The coiled strips are pressed into the jacket tube subject to appropriately intense prestressing. However, durability tests conducted on this type of monolithic catalyst with a metal carrier have shown that, in view of the severe stresses to which the monolithic catalyst is subjected to in a motor vehicle, the rough points of contact between the metal strips are not sufficient to prevent, as a result of the exhaust gas pressure, outward displacement or telescoping of the matrix from the jacket tube. By way of remedy, specially designed metal pins were driven from one side of the jacket through the matrix to the other side of the jacket and welded to the jacket tube in order to prevent lateral escape of the exhaust gas from the monolithic catalyst. This design of monolithic catalysts with a metal carrier is disclosed in Finnish patent application 896 294. However, in stringent tests under conditions approaching those of practical operation various versions of this method also offered only imperfect protection against the aforementioned failures, since the ferritic steel used begins to soften at temperatures between 550.degree. and 900.degree. C., depending on the exact composition of the material, and the vibrations and gas pulsations in the exhaust system cause the matrix in the region of the retaining pin to be loosened and detached.
Moreover it will be readily appreciated that the smaller the cellular density of the metal carrier, the lower is the retaining effect of the metal pin. Hence, for reasons of durability, low cellular densities of about 30 cells/cm.sup.2 are barely practical.
Furthermore, the fitting of a retaining pin increases both the loss of pressure and the weight of a monolithic catalyst with a metal carrier of this type, thus reducing its advantage by comparison with respect to the other type of monolithic catalyst with a metal carrier.